Pulse position modulation (PPM) is a form of signal modulation in which the timing of pulses is modulated to transmit information. PPM is primarily useful for optical communications systems, where there tends to be little or no multipath interference, and particularly in vacuum where there is no refractive index dispersion.
In one prior art technique for generating an optical PPM signal, the pulse timing for each pulse is modulated in a waveguide channel with a linearly chirped Bragg grating built into the waveguide channel. The modulation shifts the pulse in time; however, the linearly chirped Bragg grating built into the waveguide channel also spreads or disperses the pulse in time. To remove the pulse dispersion some form of dispersion compensation must be performed. In the prior art the pulse dispersion has been performed using separate temperature-controlled devices such as fiber Bragg gratings (FBGs), dispersion compensating fiber, volume hologram chirped gratings or surface gratings. However it is difficult to fabricate a linear chirp grating in any of these separate devices that is identical to the grating which generated the chirped pulse. For example, nonuniformities in the glass used to fabricate the FBGs or volume holograms are difficult to avoid, which makes it difficult to fabricate FBGs or volume holograms with the required degree of accuracy. Temperature control helps, but adds bulk and still may not provide enough accuracy. Also sending the signal backward through the same grating could eliminate the dispersion but would simultaneously eliminate the PPM shift.
In addition, these prior art approaches do not address the issue of compensating for pulse dispersion while simultaneously allowing the position of the pulse to be modulated, as required for PPM signals.
What is needed is a system and method for accurate pulse dispersion compensation while allowing pulse position modulation to be maintained. A system and method for PPM is also needed which is compact enough for satellite applications. The embodiments of the present disclosure answer these and other needs.